Board index Flathead Power-Technical Questions, Answers, and Suggestions K-Models KH piston clearance

KH piston clearance

Moderators: Curt!, Pa

Post Tue Mar 21, 2006 12:33 pm

Posts: 54
Does anyone have a factory documentation or service letter that specifies the piston to wall clearance for a 1956 KH?

Post Tue Mar 21, 2006 12:42 pm

Posts: 587
Location: Crewe, Great Britain

Hi, Hans

from H-D Shop Dope sheet # 328, June 4th, 1952, all models except 125 should be fitted .001" to .002", piston measured front to rear at the bottom of the skirt

from H-D Shop Dope # 346A, June 1st 1954, KH: .001" to .0015", cylinder measured 1/2" from top of bore


Post Wed Mar 22, 2006 8:00 am

Posts: 54
These are very small clearances. I find it surprising that my KR and XLR manuals give .007" for KR and .006" for the XLR. Is it normal that HD gave so much extra clearance for the racers? Anyone knows the clearances for WLDR and WR?

Post Wed Mar 22, 2006 10:49 am

Posts: 240
Location: Kirkland, WA, USA

Hi Hans,

As you know I just put my KH back together again…. I have found over the years, yes even with bad breather timing, that it will run without seizing from .oo2 to .006 with aftermarket cast pistons. This is the key though, as after market pistons I have been told have a bigger expansion rate than the stock pistons. I just set mine up between .003 and .004 as we had to re-hone, and it was on the tight side the first time on this bore. Since I don’t know how you run her, I would suggest that .003 would be a safe place to start. Stock pistons from the factory were recommended at .001 to .002 .

Over .006 and it is likely time for a new bore. Most on the forum would recommend that you have the top and bottom of the cylinders torque plated when you have them bored to get the best possible set up, ending up with a round bore. Making the plates takes time, but may be well worth it.

It is still raining on my days off, so my bike sits in the garage waiting for a test ride. Pouring today, but it is Seattle so what do you expect? Of course when I am working, it is beautiful!

Best Regards,

Post Wed Mar 22, 2006 12:10 pm

Posts: 1050
Location: Greenbackville, Virginia, USA
Hans, my 54KH is at 0.003" with v-twin pistons & Totalseal rings with no problems. Definately torque plate. G 8)
The reason people blame things on previous generations is that there's only one other choice

Post Wed Mar 22, 2006 8:13 pm

Posts: 149
Location: calif
yes kr and xlrs need more clearance. as they r run harder and more heat. been duing it for years. not all pistons need same clearance cast is diffrent than forged. with kr xlr xr 750 when they r cold should not rev much till motor warms up. if u use v twin listen to fellow who had experance. cant beat that. if possible check with piston mfg. they will give u bad kieth black {silverlite} dont make them the r xlint pistons. i have some new kr pistons. but have used v twin they seem to work well. best to listen to experance and piston mfg. specs. best to u.

Post Fri Mar 24, 2006 4:08 am

Posts: 54
Are the HD race pistons (KR, XLR) different from the stock pistons? Are they possibly forged?

Post Fri Mar 24, 2006 12:18 pm

Posts: 149
Location: calif
hi kr xlr pistons r cast. i have new ones. on box part # 22073 65ra casting # on piston 22278-52 for kr. in fact jerry branch said once stock k pistons if fine due to low comp. also rings were one piece.

Post Mon Apr 03, 2006 10:35 am

Posts: 55
Location: west coast
Are these "new" pistons cam ground? The "old" OEM pistons were cam ground, and could expand with heat, with out seizing. If you get .006 clearance, it will be noisy. I would go for, at a minimum, .002

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